It is the first time that tree density has been linked with asthma. There has been around 160 per cent increase in asthma cases globally in the last two decades. The rise in asthma cases is believed to be linked with reduced exposure to bacteria that leaves the immune system underdeveloped.

The findings, reported in The Journal of Epidemiology and Community Health, suggests that a leafy suburban lifestyle goes some way to protecting children against the disease.

“Street trees were associated with a lower prevalence of early childhood asthmas,” the team led by Dr Gina Lovasi wrote in the journal.

The analysis, based on data on asthma rates across New York, found that children in the greenest streets were least likely to develop asthma, although the degree of leafiness was not correlated with the number of more serious asthma attacks that led to hospitalisations. In New York City, asthma is the leading cause of admission to hospital among children under 15.

Researchers from Columbia University in New York found that asthma rates among four-to five-year-olds fell by almost a quarter for every 343 extra trees per square kilometre in an urban area, the report said.